Abstract

Drone technology has proved to be an invaluable tool for geoscientists. It is now possible to map, in some cases down to the centimeter scale, previously inaccessible outcrops. For exposures that have been examined, views from the air and photogrammetric models make more rigorous interpretations possible and, in some instances, bring to light new stratigraphic, structural and facies relationships.

Examples presented here include two quarry high walls (from Colorado and Mississippi) and a bluff section along the Tombigbee River of Alabama (Figs. 1-7). All the exposures were rapidly surveyed for later model building, structural measurement, and interpretation. Orientations of beds, faults, and joints were measured solely from the 3D models.

Scenarios for planning and gathering aerial images are examined with examples. Vertical outcrops (bluffs and high walls) require manual flight operations in many cases as opposed to rock outcrops with more aerial extent and relatively little relief. Two main approaches for surveying bluffs and highwalls are: manual flight mode relying on the drone’s obstacle avoidance sensors to mitigate collisions with the rock face and the course lock mode which allows the drone to fly a straight path regardless of its orientation. These methods are often used in combination.

Resolution of the 3D models vary from the type of drone used, required detail needed for a given project and time constraints for data collection. Quarries present unique hazards such as heavy vehicle traffic, potential high-wall rock fall, and various terrain issues (unstable rock piles or standing water). River outcrops may only be accessible by boat, may not have much bank area for operations, limited visibility depending on bank geometry, and in many cases will have both rock and vegetation overhangs to fly around or under. In both scenarios, surveys can entail lengthy flights over water or inaccessible areas should the drone run into trouble and require recovery. Flights with such varying conditions require manual operation of the drone and constant vigilance for hazards when the drone up to a thousand or more feet away.

Project goals determine how 3D models, orthomosaics and surface models are interpreted. Film or digital images and montages provide only apparent dips and limited stratigraphic details. 3D photogrammetric models allow the accurate measurement of strikes and dips, bed thicknesses and facies variations in three dimensions. Because drones allow for safely viewing and measuring inaccessible outcrops up close many new and unexpected relationships are regularly documented.